Advances in Dental Implant Positioning Techniques and Their Clinical Implications ()
1. Introduction
For many edentulous patients, implants remain as a more reliable, functional and aesthetic alternative compared to the traditionally used prosthetic appli- ances. An increasing demand for implant-retained restorations has resulted in a need for new techniques to ensure proper implant positioning.
The definition of a successful implant has been refined from just achieving effective osseointegration to a precise positioning and good prosthetic outcome. To achieve such outcomes, use of appropriate treatment planning is increasingly becoming popular. Suitable treatment plans aid in placing the implants in an optimal position, as dictated by the function and aesthetics of the definitive restoration, and to avoid biomechanical complications and failure [1] . The application of computers to various fields of dentistry has led to many advances [2] . One among them is computer-guided implant placement [3] . The aim of this work is to critically review the evolution and clinical implications of various dental implant placement techniques with a special focus on computer-guided implant placement techniques.
2. Materials and Methods
3. Discussion
3.1. Complications of Improper Implant Positioning
Most of the reviews illustrated a strong correlation between improper implant position and the marginal bone loss (BL). There are two main explanations found in the literature for the BL. One is disuse atrophy due to subnormal mechanical stimulation [4] and another reason is microgap coming close the bone [5] .
It has been shown by many authors [5] [6] that, next to BL, incorrect angulation of the implant often leads to periimplant bone destruction and mechanical complications. As early as 2004, Buser et al. reported that the labial improper placement not only leads to labial bone thinning but also lingual BL and gum recession [7] . It is notable that thinning of the lingual bone, as demonstrated by Tarnow and Eskow (1995), results in emergence problems [8] .
Placing the implant both too close or too far apically often cause resorption. The former is investigated in detail by Esposito et al. (1993) which explained the resorption of the interproximal alveolar crest [9] and the latter is studied by Buser et al. (1991) that showed bone resorption [10] . A study by Thilander et al. (1999) has established that the incorrect implant placement near the adjacent teeth not only leads to interproximal BL but also causes tooth vitality loss [11] .
Various works also report injuries due to serious complication of mandibular implant osteotomy. For example, Bartling et al. (1999) have reported that the inferior alveolar nerve injury can lead to altered sensation of the lower lip [12] . Vazquez et al. (2000) also have explained that such injuries can take place if the magnification factor on the panoramic radiograph is misinterpreted [13] .
Similar injuries of mental nerves also frequently reported. Bartling et al. (1999) studied such injuries and demonstrated the importance of correctly locating mental foramen, which can vary from the mandibular canine to the first molar, during surgical procedures in the lower premolar area [12] . Later Grenstein et al. (2006) illustrated the use of CT scans for efficiently detecting the mental foramen than conventional 2D radiographs [14] .
Although not very common, lingual bundle injury due to perforation of the lingual cortex is also reported during the incorrect mandibular implant placement. Kalpidis et al. (2004) established that subsequent profuse bleeding and progressive hematoma expansion may cause displacement of the tongue and the floor of the mouth, leading to fatal airway obstruction [15] .
Other rare but serious complications noted in the literature include perforation of the nasal or sinus floor, injury of the salivary glands [16] , and mandibular fracture in cases of osteoporotic and atrophied mandibles [17] .
3.2. Quest for Optimal Position
It is commonly accepted that the ideal implant distribution and placement are critical to ensure the optimal mechanical and aesthetic outcomes of definitive restorations, and to enable patients to maintain proper hygiene. Various works have discussed about what is ideal placement in detail and they are summarized in the following Table 1.
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Table 1. Criteria for ideal implant placements.
Not only just the implant placement but also their distribution is studied in details by various experts. The antero-posterior implants and implant- and tissue-supported overdentures are few cases where both the placement and the distributions become critical. The former is studied by Palmqvist et al. (2007) who have emphasized the importance of allowing equal distribution of the load over a wide area with minimal cantilever length [26] . Jivraj S. et al. (2006) enunciated that the implants in the case of implant and tissue-supported over dentures, must be placed such that a constructed bar has a straight-line connection between the implants and does not encroach on the palatal/lingual denture-bearing area with adequate space for the clip. They have further explained the importance of placing more implants when the clinician anticipates high force on the implants from the opposing occlusion to share the load [27] .
In addition with satisfying these ideal conditions, it is commonly found in literature that in the case of anterior mandible implant, all preventive measures should be taken before, during, and after implant placement. Experts commonly agree that the surgeon should be skilled and aware of the regional arterial anatomy. Proper treatment planning should also be ensured through radiographic and clinical evaluation of the osseous morphology and finally an implant of the correct angulation and length should be selected.
3.3. Importance of Treatment Planning
The careful evaluation of the surgical site and the interaction between the restorative dentists and dental surgeon leads to appropriate treatment planning. There are wide varieties of traditional tools for treatment planning available. They include panoramic, cephalometric and periapical radiographic films, diagnostic wax-ups, and articulated study models [28] [29] . Other diagnostic aids like photography and the ridge-mapping (RM) technique is very useful for assessing the implant bone site. Direct measurement of ridge size remains as the most accurate diagnostic tool but can only be performed during surgery. Ridge size can also be measured using linear tomography or cone-beam CT, but these techniques often lead to under or over-estimation respectively [30] [31] .
Being an invasive procedure, RM is difficult to be used in the cases of shallow labial, lingual vestibules. Moreover, RM cannot be used to verify the actual position of the inferior alveolar nerve.
Most of the times the information pertaining to treatment planning required to be collected before surgery to avoid unexpectedly complication surgeries (such as bone harvesting) and to increase treatment predictability. Conventional 2D radiography remains as an important treatment tool but before using but before using one has to be aware of its diagnostic limitations in terms of magnification, distortion, setting errors, and position artifacts [32] . For example, 2D radiographs do not show the lingual anatomy or provide the complete 3D information about the dental arch [33] .
Placing the optimal number/size of implants to achieve optimal prosthetic outcomes remains as the goal of preoperative implant treatment planning. This necessitates a thorough knowledge of the patient’s 3D bony anatomy. This in turn enable the clinician to evaluate the suitability of the remaining bone for implant placement, regarding whether there is sufficient bone volume (BV), bone density (BD), and an appropriate axis of orientation [32] .
The use of CT in implant dentistry has tremendously improved treatment planning. CT can be used to identify dental-related jaw anomalies [34] and to determine the optimal implant size and angulation, to avoid injury of critical structures [22] [24] . By allowing visualization of the scanned jaw bone in cross- sectional, axial, and panoramic views, CT enables the precise planning of implant placement relative to the bone and future prosthesis, especially when a radiographic template is used [34] . However, the 2D nature of the CT scan still requires the clinician to integrate the images in his mind to gain the desired information in 3D [35] . However the recent advances in computational methods offer promising solution to overcome this barrier.
3.4. Interactive Software Programs
There are number of clinical computer applications developed to allow clinicians to obtain 3D models and plan virtual situations [36] . These programs are increasingly used as tools for implant diagnosis, planning, and treatment execution. In addition to their use with imaging techniques, interactive software programs are used to construct surgical templates for transferring the planned treatment to the patient’s mouth [37] . Various commercial software programs are available, most of which display an axial cut, a panoramic cut, and multiple bucco-lingual (parasagittal) cuts [38] . These tools often provide a reformatted 3D image [39] , in which the bony structures are visualized with the possibility of incorporating other anatomical structures or soft tissues [37] .
3.4.1. Measuring Bone Density
Apart from offering plethora of visualizing tools, technology enables clinician to measure bone density (BD). As BD corresponds to bone quality, it is crucial for the success of dental implants [40] . Site-specific measurements of BD provide information for evaluating how long the interval between the first and second stages of surgery should be [41] . An accurate preoperative measurement of the BD helps to prevent the placement of implants in areas of poor quality. Use of CT with interactive software is a viable and accurate method to measure BD [42] . Recent efforts in oral imaging have focused on developing automatic tools to measure BD by x-ray absorption methods [43] . Owing to such efforts, CT images containing BD data in DICOM format are now available, enabling different software programs to measure this parameter [42] .
3.4.2. Estimating Bone Graft Volume
Software programs in implant dentistry can also be used to evaluate the BV. A lack of sufficient BV frequently precludes conventional implant placement, necessitating the use of augmentation procedures [44] . Detailed preoperative information about the needed BV can aid the surgeon in determining the best donor site [45] , estimating the volume of xenographic bone substitute required [46] , and minimizing the surgery duration, risk of complications, and patient expense [47] . Through software programs, the BV needed to be augmented can be calculated from cross-sectional CT images [48] .
3.5. Recent Developments in Implant Surgery: Applications of Modern Technology
Recent technological developments paved way for more advancements in restoration and implants. Few of such major areas include: flapless implant surgery, immediate restoration and zygomatic implants.
3.5.1. Flapless Implant Surgery
The traditional implant protocol set by Branemark required a few months for osseointegration of the end osseous implants before the definitive dental prostheses could be installed [49] . With the advent of modern technology, including CT, interactive software programs, and CAD/CAM-fabricated surgical guides, the development of “flapless” implant surgery has been made possible [50] . The goal of this minimally invasive surgical technique is to reduce treatment costs and patient healing time. Implants placed without flap elevation show increased osseointegration and periimplant bone height compared to implants placed with flap elevation [51] . Moreover, the resulting small, clean, closed wounds heal quickly with little scar formation, compared to large open wounds that heal slowly and with significant scarring.18 D.C. Sabiston and H.K. Lyerly, Textbook of 
surgery
, Saunders, The biological basis of modern surgical practice, Philadelphia (1997) p. 207-220.
Flapless implant surgery is a popular approach with numerous advantages, including improved patient comfort and healing, decreased surgical time, and the ability to resume normal hygiene procedures immediately after surgery. However, this approach is only indicated when the surgeon is confident that the underlying osseous anatomy is ideal relative to the planned implant size, and that its 3D position in the alveolus. Without these conditions, many problems may arise, including injury of unseen vital structures, thermal damage, malposed angle or depth of implant placement, and inability to contour the osseous topography appropriately to facilitate restorative procedures [52] .
3.5.2. Immediate Restoration
The precise implant placement offered through the application of modern technologies to dentistry has allowed immediate loading with prefabricated final restorations [53] . For example, the recently introduced “Immediate Smile” treat- ment protocol allows the simultaneous placement of endosseous implants and a CAD/CAM-guided, immediately loaded, definitive prosthesis [54] . Its viability is supported by its accuracy, which allows the virtually planned 3D model to be transferred to the surgical template, the implants to be placed, and the prosthesis to be attached immediately after abutment connection [53] .
3.5.3. Zygomatic Implants
The use of CT data and interactive software program for treatment planning has made the management of compromised cases much easier and more predictable. For example, sinus pneumatization in a long-term edentulous patient can complicate implant placement. Maxillary sinus-lift procedures are frequently performed to create adequate BV for predictable implant placement [53] . However, these procedures have many disadvantages, including donor site morbidity, long operation time, high cost, and invasiveness [56] . CT is an extremely useful tool for evaluating the bone trajectory in the posterior maxilla. By using information from the CT scan, the implants can be inclined to avoid the maxillary sinuses, and alternative procedures (e.g., using existing anatomical sites) that offer reduced morbidity and minimal invasion of the existing structures may be used [27] .
The zygomatic implant is a promising alternative for the management of the severely atrophied maxilla. Zygomatic implants can be placed to engage the zygomatic bone infero-lateral to the orbital rim, to provide anchorage for a fixed prosthesis in conjunction with anterior implants [57] . However, due to the anatomy of the zygomatic bone and the implant length, placement of a zygomatic implant poses a challenge for prosthodontists [58] . Based on computer- aided preoperative treatment planning with CT data, stereolithographically produced customized drill guides have been developed to determine the location, angle, and implant insertion depth. This approach provides a link between the planning and actual surgery, by transferring the virtual treatment plan accurately to the patient’s mouth [58] . For optimal implant positioning, it is mandatory that the treatment plan be precisely transferred to the patient’s mouth [59] . Two main problems still affect the accuracy of zygomatic implant placement in a severely atrophic edentulous maxilla. The first problem is the stability of the drill guide, which must rest on the underlying tissue. An unstable surgical guide will negatively affect the accuracy of translation of the preoperative treatment planning to the operative field [13] [19] C.C. Galanis, M.M. Sfantsikopoulos, P.T. Koidis, N.M. Kafantaris and P.G. Mpikos, 
Computer
methods for automating preoperative dental 
implant
planning: 
implant
positioning and size assignment, Comput. Methods Programs Biomed.86 (2007), pp. 30-38. Article | 
PDF (665 K)|View Record in Scopus|Cited By in Scopus (6). The second problem is the manual placement of the implant; currently, guidance is only provided for drilling and not for implant placement [58] .
3.6. Computer-Guided Implant Placement
The technology is playing a vital role also in optimally placing implants. There are number of issues with freehand implant placements and various tools have been proposed to overcome them.
3.7. Computer-Guided versus Freehand Implant Placement
In freehand implant placement, execution of the treatment plan is guided by mental navigation [58] , which inevitably leads to inaccuracies in implant positioning. For example, a study of implant positioning in the posterior maxilla found that only 20% of freehand-placed implants could be categorized as “ideally placed” [60] . The use of conventionally constructed surgical templates cannot ensure placement accuracy if the created holes or slots do not guarantee the correct axis of the intrabony implants, and they cannot ensure parallelism when multiple implants are placed [61] .
These templates only guarantee that the entry point of the drill is at the ridge crest, which later is translated as the position of the implant head [33] . However, it is misleading to judge the position of the implant through the location of its head at the bone crest. Additionally, the parallel appearance of multiple implants on 2D radiographs does not mean that they are actually parallel. Finally, nearly five drills are used in the conventional freehand protocol, any of which may deviate from the original plan, resulting in unguided implant placement in the prepared implant site [54] .
In computer-guided implant placement, the accuracy can be judged by comparing the postoperative outcome to the recorded preoperative 3D plan [62] . However, in freehand implant placement, there is no accurate preoperative plan to compare the postoperative outcome against [50] . Even when CT scans and software programs are used to create a preoperative treatment plan, the freehand placement will always deviate from the ideal position in the preoperative plan [63] . Moreover, although researchers agree on the value and necessity of accuracy, there is no clear consensus as to its definition or the parameters used to evaluate it [58] . Furthermore, a universal and absolute value (in millimeters) for an “acceptable” deviation cannot be defined, because even the smallest deviation might be significant in some clinical situations (e.g., nerve injury) [27] .
3.7.1. Tools for Computer-Guided Implant Placement
Three tools for computer-guided implant placement have been reported in the literature: robots, navigation, and CAD/CAM-generated stereolithographic surgical guides (SSGs). Robots are accurate tools to transfer the treatment plan to patients; however, their high cost and low availability limit their use [62] . Navigation systems permit improved precision of insertion in terms of the position, angulation, and depth of implant placement compared to conventional freehand placement [64] . Protocols using a navigation system reportedly result in similar accuracies compared to protocols using CAD/CAM-developed SSGs [65] . Park et al. (2009) compared the accuracy of 45 implants placed on a manikin through a CAD/CAM-fabricated surgical template to freehand placement [40] . The lateral deviation and angulation of the implant axis were reduced in the CAD/ CAM-guided implants compared to the freehand-placed implants. Accuracy was improved when the guidance extended to the implant placement itself and not the drilling guidance alone. The horizontal deviation was minimized, the implant angulation was more accurate, and the implant depth was fully controlled by specially designed stoppers on the drills and the implant holder. Nickenig et al. (2009) achieved similar results in their study of 23 implants in 10 patients [62] .
Historically, implants were placed without the use of surgical guides; however, increased demands for more appropriate implant positioning has led to surgical guide development [56] . Surgical guide fabrication begins with a diagnostic tooth positioning, either through a waxing, denture tooth arrangement, or duplication of the preexisting teeth or restorations [66] . SSGs are precise metallic guides that are closely matched in diameter to the drills or implants [63] . They are fabricated through CAD/CAM technology and rapid prototyping machines [67] . Resin is laser-polymerized layer-by-layer at 1-mm thickness, which corresponds to the slice interval in the CT-formatting process. Usually, they are made in sets of three, each incorporating metal sleeves (5 mm in height) and different diameters corresponding to the diameters of the surgical drills [34] . They can be fabricated to fit to bone, teeth, and/or mucosa, per the surgeon’s request [54] .
The surgical guide can guide both the drills and the implant placement. Only two drills are used (the pilot and the final drill), which have depth-control stops and are stepped such that they can be guided through the same guiding tube. Because the same drill can be used to drill osteotomies of different depths, the surgical guide is fabricated so that different guiding tubes are placed at different heights to adapt to the depth difference of the osteotomies. Drilling is performed until the drill is blocked by the depth-control stops. By using the same surgical guide, the implants can be placed, and the depth of implant placement can be secured through the specially designed, differently sized implant holders [54] .
3.7.2. Benefits and Limitations of Computer-guided Implant Placement
The main benefit of CAD/CAM-guided dental implant planning and placement is that it allows thorough preoperative diagnostics and a more predictable implantation procedure [58] . It reduces the risk of damage to adjacent structures [56] , and permits the implementation of restorative goals through accurate presurgical planning [34] . In some cases, use of these systems may allow bone augmentation procedures to avoided (e.g., by utilizing present bone) or flapless surgery to be considered [52] . Use of an accurate surgical guide permits the implants to be placed precisely into planned positions, which may allow the immediate delivery of a prefabricated final prosthesis [69] .
However, computer-guided implant placement has some [36] limitations, including a high treatment cost (i.e., due to the software purchase, radiographic template and SSG fabrications, and CT scan) and the high radiation dose of the CT scan [36] . The radiation risk can be reduced by lowering the dose output of the scanner [70] and using modern scanners, which produce lower radiation doses. Some authors [62] [71] [72] recommend the use of cone-beam CT, which can decrease both the cost and radiation dose.
Under/overestimation of the BV during CT-driven treatment planning and virtual implant planning may reduce the predictability of implant positioning. This problem might by overcome by increasing the resolution of the CT data and using sufficient exposure during scanning [31] . An inaccurate CT scan [63] and lack of stability or reproducibility of the radiographic template during CT will lead to improperly scanned data for treatment planning, with subsequently incorrect clinical implant positioning.
A poorly fitted or unstable surgical guide will affect the precision of implant placement. Poor access in posterior areas can result from the large surgical guide thickness or guiding cylinder height. Limited interocclusal space in the posterior segments is problematic and may make drill insertion through the surgical guide impossible, necessitating freehand implant placement. Poor visibility during drilling can make it difficult to verify the proper drilling depth and perform instrumentation, especially in posterior areas [69] . Heat generation in the drill, due to the enclosure of the drill within the guiding cylinder of the surgical guide, can also lead to implant failure.
4. Conclusions & Results
Based on the available data, CT scanning and interactive software programs for the treatment planning and fabrication of CAD/CAM-based SSGs appear to be viable and promising tools that may improve the predictability, safety, and precision of implant placement compared to conventional freehand placement. The high accuracy of computer-guided implant placement may facilitate the clinical management of complicated cases. Moreover, the high accuracy and precision of transferring the treatment plan to the patient’s mouth makes the immediate delivery of a prefabricated final prosthesis a reliable option.
However, computer-guided implant placement is not without limitations. It is a technique-sensitive procedure, involving the creation of studying models, diagnostic wax-ups, and radiographic templates, CT scanning, interpretation of CT data, treatment planning, and the fabrication and stabilization of a surgical guide in the patient’s mouth during surgery. An error at any of these steps may affect the accuracy and precision of implant placement. Although computer- guided implant placement is a promising technology, its performance must be critically evaluated, particularly because this technique is already commercially available and used by many clinicians. Given the limited data, relatively short observation periods, and lack of randomized controlled trials available in the literature, prospective clinical studies with long-term follow-ups should be performed. Such studies should strive to improve the systems and procedures with respect to their accuracy, predictability, and reproducibility of implant placement, as well as the surgical and prosthetic outcomes.